How did foreign countries treat POWs in World War 2?

Answer 1

Of the major participants in WWII, only Japan and the Soviet Union had not signed the Hague and Geneva conventions concerning treatment of POWs (and civilian internees). Thus all other states were bound by it and by and large all other states abided by it too, at least in relation to each other.

There were numerous offenses of course, committed by all participant states.

The Geneva states did not feel bound by the convention in relation to states who had not signed (Japan, Soviet). The Western allies chose to respect the convention nonetheless (in relation to Japan). The Germans chose to not respect it (in relation to Soviets), which led to systematic use of slave labor with fatal outcome for millions of Soviets.

The Soviet Union and Japan had no officially proclaimed view on POW, and treatment of such was subject to wartime needs and local discretion. Normally, POWs were used for labor tasks. Few POWs survived in their care.

All participant states also interned civilians, on ethnic and political grounds, and robbed them of recourse to the law. Thus against their own peoples, all states committed offenses against the conventions too.